1. Field of the Invention
The invention relates to a DC-DC converter with auto-switching between pulse width modulation (PWM) and pulse frequency modulation (PFM), and more particularly to a DC-DC converter, which has auto-switching between PWM and PFM, is applied to an organic light emitting diode (OLED) display and selects a PWM mode or a PFM mode according to display data of the display.
2. Related Art
An organic light emitting diode (OLED) display displays a frame using light emitting elements. The OLED display also displays the frame according to the brightness differences generated by different currents flowing through the pixels. The current of each pixel varies with the variation of the displayed frame, the difference between the maximum power consumption and the minimum power consumption of the OLED module is significantly great. It is a great challenge to provide the stable power for the OLED module, such as the power with the small ripple, without lowering the efficiency.
The DC-DC converter has two modulation methods including pulse width modulation (PWM) and pulse frequency modulation (PFM). As for the efficiency, PWM is suitable for the condition of great current loading, while PFM is suitable for the small current loading. Most DC-DC converters use the single modulation method, and some DC-DC converters judge the switching between PWM and PFM according to the magnitude of the current loading. However, it is not an easy issue to switch between PWM and PFM according to the magnitude of the current loading.
The technology of auto-switching between the PWM and PFM operation modes using in the DC-DC converter will be described in the following. Referring to FIG. 1, the auto-switching DC-DC converter 10 disclosed in U.S. Pat. No. 7,173,404 includes a starter 102, an auto PWM/PFM controller 103, a PWM controller 106, a PFM controller 108 and a power switch unit 110. The load 112 in this drawing may be an OLED module of an OLED display.
The starter 102 transmits a starting-enabling signal to the auto PWM/PFM controller 103 to enable the auto PWM/PFM controller 103. Thereafter, the PWM controller 106 and the PFM controller 108 respectively transmit feedback signals to the auto PWM/PFM controller 103. The auto PWM/PFM controller 103 generates a select signal according to the feedback signals of the PWM controller 106 and the PFM controller 108, and outputs the select signal to the PWM controller 106 and the PFM controller 108. So, the auto PWM/PFM controller 103 enables the PWM controller 106 or the PFM controller 108 according to the select signal. The enabled controller (PWM controller 106 or PFM controller 108) outputs a control signal to the power switch unit 110. Thus, the power switch unit 110 switches and outputs a DC power to the load 112 according to the control signal. Meanwhile, the load 112 also transmits another feedback signal to the PWM controller 106 and the PFM controller 108.
Therefore, the auto-switching DC-DC converter 10 controls the power switching in a close loop manner. In general, the close loop control mode is more complicated. So, the cost is higher.